Glacier-dammed ice-marginal lakes of Alaska

Abstract

The climate across Alaska is changing, as are the melting and other dynamics of glaciers and their lengths, widths, thicknesses, and masses. One may reasonably expect that terminus and surface ablation of Alaskan glaciers would impact the occurrence and sizes of glacier-dammed lakes (GDLs) impounded by many of these glaciers. An individual lake and its damming glacier may have unique dynamics, exhibiting chaotic variations not directly attributable to climate change. Statistical analysis of a large GDL population, however, indicates that changes may be linked directly to shifting climatic conditions, thus providing an empirical basis for quantitative numerical models of future behavior. In this chapter we show that the rapidly changing distribution of glacier-dammed lakes is neither simple nor homogeneous, but that lake changes may be amenable to modeling. These lakes, their impounding glaciers, and their changes through time can be detected in satellite imagery. Presented here is research enabled by the GLIMS initiative, and the Terra/ASTER and Landsat programs. Archived imagery facilitated review and change analysis of 538 previously mapped GDLs across Alaska, U.S.A. All glacier perimeters within the study area were also reviewed for additional lakes forming since the prior survey in 1971. Change analyses of the combined lake population over time found nonuniform distributions of lake losses and gains from 1971 to 2000 across Alaska. Detected changes appear less related to elevation, latitude, or maritime influence than to the complexity, origin and terminus types of damming glaciers, the aspects of their ice dams, possibly the topographic gradient below and near the lake, and possibly the rate of recent temperature increases. The Copper River Basin (CRB), for example, has recorded low rates of atmospheric warming relative to adjoining areas over the past 50 years, and it has retained and developed the greatest proportion of GDLs. A concurrent detailed remote-sensing and field observation study of the CRB highlighted the dynamics of individual GDLs and their damming glaciers. Whereas there is no single typical or representative lake, Iceberg Lake, in the far eastern Chugach Mountains, provides an example showing how a climatic shift to warmer conditions may result in diminishment and even disappearance of these lakes. Iceberg Lake was comparatively stable for at least 1,500 years, but responded to > 100 years of thinning of its damming glacier with the initiation of episodic glacier lake outburst flood (GLOF) drainages every year or two, starting in 1999. The thinning of the damming glacier, in turn, is partly a response to a moraine-dammed lake that has formed since the 1970s at the terminus of the trunk (Tana) glacier, catalyzing disarticulation of that terminus and causing a furtherance of thinning of Iceberg Lake’s damming glacier. The entire system including the Bagley Icefield, outlet glaciers, multiple GDLs (Iceberg Lake among them), and oraine-dammed lakes, is dynamically complex; some parts appear to be responding to climate shifts, and other parts may be displaying intrinsic unsteadiness of flow (including a surge/waste cycle of Bering Glacier). At the same time as known lakes have been diminishing or disappearing, in recent decades at lower elevations, GDLs tended to form and persist at higher elevations.